Now bear in mind that I HATE this repair technique... only do it if you 'promise' to find some steel for a future, permanent repair:

window screen is easily formed to about any shape you desire. Grind down the inner side of the fender, and use fiberglass to adhere to the screen to the fender. When the screen is firmly bonded to the steel, then lay fiberglass on the screen. ( I've seen bondo used also, but fiberglass is more nearly a competent repair ). Sand it down, lay on a layer of bondo, and you're done.

2nd suggestion, and I recently got HAMMERED for a similar suggestion: ( heh heh ).. this might be a good case for a hammerform mold. Simply put, you make a form outta MDF then hammer some 20GA around it till it has the shape you want. There is a little more to it than that, but that is the basics.
( yeah, I recently bought a Ron Covell video, and I'm probably over impressed.) Look, if Ron can make an entire bugatti fender from a combination buck/hammerform then this guy might be able to make a fender patch.

they're just suggestions anyway.